CVE-2015-6526 in Linux
Summary
by MITRE
The perf_callchain_user_64 function in arch/powerpc/perf/callchain.c in the Linux kernel before 4.0.2 on ppc64 platforms allows local users to cause a denial of service (infinite loop) via a deep 64-bit userspace backtrace.
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Analysis
by VulDB Data Team • 06/13/2022
The vulnerability identified as CVE-2015-6526 represents a critical denial of service flaw affecting Linux kernel versions prior to 4.0.2 on ppc64 architecture platforms. This issue resides within the perf_callchain_user_64 function located in arch/powerpc/perf/callchain.c, which is responsible for handling performance call chain information for user-space applications. The flaw manifests when the kernel processes deep 64-bit userspace backtraces, creating a condition where the system enters an infinite loop that can completely freeze the affected system. This vulnerability specifically impacts systems utilizing powerpc64 architecture where performance monitoring and debugging features are active, making it particularly concerning for production environments that rely on kernel performance analysis capabilities.
The technical root cause of this vulnerability stems from inadequate boundary checking within the perf_callchain_user_64 function's implementation. When processing deeply nested call stacks from user-space applications, the function fails to properly validate the depth of the backtrace before attempting to traverse the call chain. This absence of proper validation allows malicious or malformed user-space processes to construct call chains that cause the kernel function to loop indefinitely, as it attempts to process an excessive number of stack frames without proper termination conditions. The vulnerability operates at the kernel level and requires local user privileges to exploit, making it a local privilege escalation vector that can be leveraged by users with minimal system access. This flaw aligns with CWE-835, which specifically addresses infinite loops in software systems, and demonstrates how improper input validation can lead to system-wide denial of service conditions.
The operational impact of CVE-2015-6526 extends beyond simple system freezing, as it can render entire systems unresponsive to legitimate user requests and administrative commands. Systems utilizing performance monitoring features such as profiling tools, debugging utilities, or applications that rely on kernel performance data collection become particularly vulnerable. The infinite loop condition can persist until manual system reboot is performed, effectively creating a persistent denial of service scenario that can be exploited by any local user with access to the system. This vulnerability particularly affects server environments, development workstations, and any system where performance monitoring is actively used. From an adversarial perspective, this flaw can be classified under ATT&CK technique T1499.004, which covers network denial of service attacks, as it creates a system-level denial of service condition that can be triggered through legitimate kernel functionality. Organizations using Linux systems on ppc64 architecture should be particularly concerned about this vulnerability as it can be exploited to disrupt critical services without requiring elevated privileges beyond local access.
Mitigation strategies for CVE-2015-6526 focus primarily on kernel version updates, as the vulnerability was addressed in Linux kernel version 4.0.2. System administrators should prioritize patching affected systems with the latest kernel releases that contain the relevant security fixes. Additionally, implementing proper monitoring and alerting for unusual system behavior can help detect exploitation attempts, though this approach is reactive rather than preventive. Organizations should also consider restricting local user privileges where possible, as the vulnerability requires local access to exploit. The fix implemented in kernel version 4.0.2 includes proper bounds checking and validation of call chain depths, preventing the infinite loop condition from occurring. Security teams should also review performance monitoring configurations to ensure that only necessary and trusted applications have access to kernel performance analysis features. This vulnerability highlights the importance of proper input validation in kernel-space code and demonstrates how seemingly benign functionality can be exploited to create system-wide denial of service conditions.